(1) Field of the Invention
The invention relates to a method for forming a poly-Si film though crystallizing an amorphous silicon film.
(2) Description of the Related Art
A poly-Si material has better attributes than an amorphous silicon material. Because of larger grain size, an electron is easier to move in the poly-Si material. That is, the mobility of the poly-Si material is higher than that of the amorphous silicon material. A transistor made of the poly-Si material has shorter reaction time than a transistor made of the amorphous silicon material. Comparing two liquid crystal displays at the same resolution, the one using a poly-Si TFT occupy less substrate area than the one using an amorphous silicon TFT, so that using the poly-Si TFT can increase the open ratio of the liquid crystal panel. Under the same brightness, the poly-Si TFT LCD can use a low-power backlight to reduce the power consumption. Therefore, the grain size of the poly-Si is an important factor affecting the efficiency of a TFT.
Presently, a poly-Si film is made on the substrate by Low Temperature Poly-Silicon (LTPS) process. The LTPS uses a excimer laser as a heat source. When the excimer laser irradiates on the substrate having an amorphous silicon film, the amorphous silicon film absorbs the energy of the laser to transform into the poly-Si film.
FIG. 1A and FIG. 1B shows a conventional method for manufacturing the poly-Si film. As shown in FIG. 1A, a nitride layer 11 is formed on a substrate 10. An oxide layer 12 is formed on the nitride layer 11. Subsequently, an amorphous silicon film 13 is formed on the oxide layer 12 by chemical vapor deposition (CVD) or sputtering. As shown in FIG. 1B, the amorphous silicon film 13 is irradiated by the excimer laser 15 to crystallize to form a poly-Si film 14. For acquiring larger grains, the energy of the excimer laser 15 should be able to almost completely melt the amorphous silicon film 13. Some non-melted grains remain on the interface between the oxide layer 12 and the amorphous silicon film 13. The melted amorphous film uses the non-melted grains as crystallization seeds to crystallize to form the poly-Si film 14.
However, the excimer laser is an pulse laser. The energy density of each pulse in the excimer laser is different from the other pulses. The larger energy density causes less amounts of crystallization seeds, and results in smaller grains and less homogeneity. According to the conventional method, the uncertain crystallizing direction makes it difficult to control the number and the location of the grain boundary. Under stable laser energy, it is necessary to reduce the crystallization energy and enlarge line beam size by an optical system to increase product output. Besides, more grain boundaries cause worse electrical characteristics of the poly-Si TFT.
To sum up, the conventional method is not capable of controlling the number and the location of the grain boundary, so that the crystallizing direction is uncertain, and it is not capable of reducing the laser energy requirement in the crystallization process, either. For reasons given above, the inventor submits a method for manufacturing the poly-Si film to improve the abovementioned disadvantages, and further, the method can be applied to manufacturing TFT.